Abiotic and biotic determinants of photosynthetic and respiratory capacity from tropical to high boreal biomes

Background/Question/Methods

The Earth System Models (ESMs) that are used to make climate projections are highly sensitive to the parameterization of photosynthetic and respiratory capacity. This stems from the fact that very little research has been done to quantify the primary mechanisms underlying these processes at global scales. Here, we measured the maximum rate of Rubisco carboxylation (V_cmax²⁵), the maximum rate of electron transport (J_max²⁵), and leaf respiration in dark (R_d²⁵), all standardized to 25°C. In total, we measured 98 species at 12 different American sites spanning 53° of latitude from the neotropics to the high boreal zone. Species sampled encompassed each of the major plant functional types simulated in ESMs. Abiotic and biotic covariate data were collected along with each measurement. We hypothesized that leaf carbon exchange capacity would be positively correlated with leaf nitrogen, as has been observed in previous studies. We also expected faster growing species to have larger capacities. Additionally we hypothesized that increases in recent temperature history would enhance leaf carbon exchange capacity (an acclimation response).

Results/Conclusions

Leaf nitrogen content was the strongest predictor of V_cmax²⁵, J_max²⁵, and R_d²⁵, with each flux showing strong increases with leaf nitrogen (p<0.05 in all cases). The fluxes were highest in annual compared to perennial plants, broadleaf compared to needleleaf plants, and, for J_max²⁵ and R_d²⁵, boreal compared to temperate and tropical plants (p<0.05 in all cases). Additional analyses suggested that the effect of climatic region may be due to differences in day length between the sites. Recent temperature history acted to decrease J_max²⁵ (p<0.05), but had little influence on V_cmax²⁵ or R_d²⁵. The response of the fluxes to soil moisture was generally weak, but an interaction between soil moisture and leaf nitrogen revealed increased R_d²⁵ response to nitrogen in wetter environments, indicating a possible co-limitation of water and nitrogen for R_d²⁵. A comparison of biotic- and abiotic-based models suggested that biotic data (i.e., plant type characteristics) are better for predicting photosynthetic and respiratory capacity than abiotic data. The mechanisms quantified here could be used to improve the parameterizations of plant carbon exchange processes in ESMs.